4.1 Configuration Registers
4.2.7 Interrupt Mask Register
There are few written sources referring to the production of glass in the Iberian Peninsula. The earliest is one by Pliny the Elder, who mentions that in Gallia and Hispania we find ‘sand subjected to a similar process’ (iam vero et per gallias hispaniasque simili modo harena temperature), which presumably refers back to the primary production of glass that he has described previously (Natural History, 36.66-67). In the seventh century, Saint Isidorus of Seville repeats Pliny’s passage (Etymologies, XVI.26; Barney et al. 2006). Hence, Isidorus’ text cannot be considered an independent source. Nonetheless, textual references such as these have provided the rationale for a large European research project exploring the isotopic signatures of silica sources along the Mediterranean coastline to probe the veracity of these literary sources (Degryse 2014). To my knowledge, no analytical nor archaeological evidence has thus far come to light that conclusively demonstrates the primary production of glass in the Iberian Peninsula prior to the establishment of Islamic al-Andalus. In contrast, at least 35 secondary glass workshops are known which date to the third to fifth centuries CE (Da Cruz and Sánchez de Prado 2012; Sánchez de Prado and Da Cruz 2014). Secondary glass-working evidently flourished in Roman Spain, implying ample supplies from the eastern Mediterranean. Remains of glass workshops become less frequent from the sixth and seventh centuries onwards and are typically associated with urban centres such as Barcelona, Cartagena and Alicante, as well as the new Visigothic foundation of Récopolis and the revived Tolmo de Minateda (Beltrán 2005; De Juan Ares and Schibille 2017a; Govantes-Edwards et al. 2020; Gutiérrez Lloret et al. 2003; Sánchez de Prado and Da Cruz 2014). As the analytical data on the glass assemblages from numerous sites throughout the Iberian Peninsula demonstrate, the glasses dating to the first to seventh centuries CE match the well-established primary glass production groups from Syria-Palestine and Egypt, thus confirming the centralised model of primary glass production and circulation during the Roman and late antique periods (reviewed in De Juan Ares and Schibille 2017a).
Recently, we have been able to refine the geographical scope and chronological sequence of glass imports between the fourth and seventh centuries CE. As with other western Mediterranean, central and northern European regions, Egyptian HIMT glass dominates the archaeo-vitreous record of Spain in the fourth and fifth centuries CE. The compositional results of the fourth- to fifth-century glass assemblage from Picola – Portus Ilicitanus in Santa Pola (Alicante), the main sea harbour of the Roman Colonia Iulia Ilici Augusta, show that 87% of the glasses are of the HIMT type (De Juan Ares et al. 2019b). Given that Picola was an important port of entry for commodities imported via the sea route, it can be assumed that
the glass assemblage is representative of developments in the Iberian Peninsula more widely. The Iberian Peninsula is not, of course, a unified historical or geographical entity, but instead comprises a number of distinct geographic regions that exhibit diverse distribution patterns of, for example, ceramics (Reynolds 2009). From the data currently available it appears that the glass assemblage from Picola nonetheless reflects broader trends in the economic environment of the Iberian Peninsula as regards the long-distance import of bulk vitreous material from the eastern Mediterranean. Only a handful of glasses of Levantine origin have been identified, unlike Visigothic assemblages where Levantine glasses are in the majority from the second half of the sixth into the seventh centuries CE (De Juan Ares et al. 2019a). The analyses of the glass assemblages from three rural Visigothic sites in central Spain identified clear changes in the supply of glass to the Iberian Peninsula in the second half of the sixth century. Egyptian glasses in the form of Foy 2.1 and Foy 2.1 high Fe prevailed during the fifth and the first half of the sixth centuries, at which point these Egyptian groups were supplanted by Apollonia-type Levantine I glass alongside the now widely recognised late antique Magby glass type with a plant ash component that is again an Egyptian glass group (De Juan Ares et al. 2019a).
The glass from Recópolis – exception to the rule or genuine trend?
It has been argued that the evidence from Visigothic assemblages pointing to a geographical shift in glass supply from Egyptian to Levantine imports does not correspond to wider developments in the Iberian Peninsula and rather represents what the authors tentatively called the ‘Reccopolis anomaly’ (Govantes-Edwards et al. 2020). The city of Recópolis, located on the Tajo in the province of Guadalajara, Castilla la Mancha, was founded in 578 CE by the Visigothic King Leovigild, and there is evidence of a continuous urban development at Recópolis well into the early seventh century CE, reflected in the expansion of the palatine complex and numerous artisan workshops (Olmo Enciso 2008). It is not entirely clear when the city was finally abandoned, since some presence after the Umayyad conquest of the region in the eighth and ninth centuries CE is still attested to (Velázquez and Ripoll 2016).
The excavations yielded evidence of several glass furnaces that were originally interpreted by the archaeologists as primary production installations, at least one of which was part of the original fabric of the city (Castro and Gómez 2008).
The vitreous materials recovered so far from Recópolis, with a total weight of about 30 kg, include glass waste and crucibles and vessels of a mostly functional and utilitarian character, with a preference for open forms. We have analysed 200
glass samples from different well-stratified and controlled contexts excavated by Lauro Olmo since the 1980s, including the workshop areas as well as some of the residential quarters. Chronologically the glass finds span the late sixth to early ninth centuries CE (i.e. into the Islamic period). The samples were selected from mostly diagnostic fragments to provide a representative overview of the different types and working debris. The colours range from virtually colourless and bluish or greenish aqua to dark amber (Gómez 2017).
The analytical data reveal the remarkable homogeneity of the glass corpus from Recópolis that separates into three main compositional groups, all of which are consistent with well-established soda-lime-silica groups of eastern Mediterranean origin: Foy 2.1, Apollonia-type Levantine I and Magby (Fig. 58).
By far the largest compositional group (i.e. 76% or 151 samples) corresponds to Levantine I glass from Apollonia. Fifteen of these Levantine I glasses have exceptionally high lime concentrations (CaO > 11%), and another 21 fragments show signs of some recycling in the form of elevated manganese levels (Mn > 250 ppm). Approximately 15% of the assemblage (i.e. 28 samples) is of the Foy 2.1 glass type, half of which has elevated iron concentrations (Fe2O3 > 1.5%). Twelve samples, representing 6% of the assemblage, have potash and magnesia levels in excess of 1.5% and roughly match the composition of Magby glass. Finally, nine samples have compositions that cannot easily be classified, due to a considerable degree of recycling.
Distribution of compositional groups identified at Recópolis (n=200) across time and space.
(a) Relative abundance of the different glass groups according to the find spot; (b) temporal pattern of the different glass groups; (c) ubiquity distribution of the compositional groups (Schibille, unpublished).
The detailed characterisation of these glass groups is beyond the scope of the present chapter, as they have all been discussed in chapters 1 and 2 on Egyptian and Levantine natron-type glasses. However, there are a number of special features that are worth pointing out. The Levantine glasses from Recópolis, for example, overall correspond closely to the sixth- to seventh-century Levantine I glasses from Apollonia, Beth She’an, Dor, Jerusalem and Sepphoris (Freestone and Gorin-Rosen 1999; Freestone et al. 2008b; Phelps et al. 2016; Tal et al. 2004).
The calcium oxide contents of the Levantine glasses from Recópolis are highly variable, ranging from about 7% to 12.5%, which may be the result of variations in the chemical composition of the sand over time and/or contaminations from the furnace environment during secondary working and recycling (Al-Bashaireh et al.
2016; Chen et al. 2021). A large piece of cullet from Arsuf (No. 3682 in Brill 1999, Vol. 1 p. 60; Vol. 2 p. 87) has a comparable composition, with lime concentrations of about 11.5%, while the high lime levels of some glass-working debris from
‘Aqir (Israel) have been attributed at least partly to contamination through furnace materials (Chen et al. 2021). The calcium contents of the high-lime (CaO > 11%) subgroup from Recópolis do not show a clear relationship with any of the other elements typically associated with contamination through fuel ash or furnace linings such as phosphorus, potassium or iron, even though magnesium oxide tends to be slightly elevated as well (Schibille, unpublished data). Otherwise the Levantine high Ca group exhibits similar features to the other Levantine glasses.
The average trace element patterns confirm that the two Levantine groups are practically identical in terms of the geochemical nature of their silica sources. The natural carbonate and feldspar impurities of the Levantine silica sources probably account for some of the variations, and the compositional spread of the Levantine glasses may simply reflect different consignments of raw glass and secondary glass-working campaigns.
The Levantine I and Levantine I high Ca groups have astonishingly low levels of other recycling markers. Manganese levels are well within the range of natural impurities in the silica source (Mn < 250 ppm), antimony is typically below 5 ppm, and lead concentrations do not usually exceed 50 ppm. This implies that the Levantine glasses appear virtually pristine, and if they have been recycled recycling was limited and highly selective, not augmenting any of the colouring elements. By contrast, the antimony and lead contaminations in the Foy 2.1 groups and Magby glasses reflect the incorporation of some recycled cullet at some point in the life cycle of these glasses. Even in the case of the Foy 2.1 and Magby categories, however, the recycling remains minimal. It must also be taken into consideration that Foy 2.1 glasses frequently have elevated transition metals above the usual background values of sand sources to start with (Ceglia et al. 2019;
De Juan Ares et al. 2019a). The high incidence of unspoilt Levantine I glass at Recópolis is remarkable; in fact, it represents the largest assemblage of Apollonia-type Levantine I glass thus far recorded in the western Mediterranean. This not only provides clear evidence that sufficient quantities of fresh raw glass from the eastern Mediterranean arrived in Spain during the sixth and possibly seventh centuries, but it also shows that the Iberian Peninsula had active commercial contacts with the eastern Mediterranean which had no parallel in Italy or France at the time. The only exception at present is Sicily (Schibille and Colangeli 2021).
These observations should come as no great surprise, as the sixth-century ceramic evidence from Benalúa (Alicante), for example, similarly contrasts with that of the northwestern Mediterranean, a fact which is perhaps indicative of a major trade link between the southeastern coast of the Iberian Peninsula and the Levantine coast, possibly via Sicily in the sixth to seventh centuries CE (Reynolds 2009, p. 117).
At Recópolis raw glass chunks were identified to be from only the Levantine I and the Magby compositional groups (Gómez 2017), suggesting that at least these two types were imported as raw glass, while glass cullet was also collected and recycled. Levantine I glass makes up virtually all the finds from workshop 1 (Fig. 58a), which is contemporaneous with the foundation of the city and which was active until the first decades of the seventh century CE (Gómez 2017). The furnaces of workshop 2 apparently operated until the end of the seventh century CE (Gómez 2017). Here, Levantine I makes up about half of the finds, while Magby glass makes its first appearance (Fig. 58a-b). The foundation of the city of Recópolis appears to have been accompanied by the establishment of a glass workshop which in the first instance obtained its raw material practically exclusively from the Levantine coast in the form of raw glass. A short time later, other glass groups (Foy 2.1, Foy 2.1 high Fe, Magby) appear on the scene and supplement Levantine I glass. The different glasses were used in workshop 2 at about the same time without extensive mixing and recycling, even though recycling is evident in some of the Levantine glasses that were also recovered from workshop 2. A simple explanation for this selective use of different glass groups is that Levantine I glass was imported as raw glass, while Foy 2.1, which is a slightly earlier glass type, may have exclusively been worked from cullet, suggesting that fresh and recycled glass may have been kept separate.
The analytical data confirm that the glass workshops at Recópolis were dedicated to extensive secondary glass-working and they had access to sufficient raw glass to minimise the necessity of recycling. We found no evidence of primary production; the three identified glass groups are all of eastern Mediterranean origin. The importance of the glass assemblage from Recópolis lies in the very
exact foundation date and the contemporaneous establishment of a secondary glass workshop. The chronological range of the different primary glass groups confirms the overall temporal developments that we have previously observed with respect to three rural Visigothic sites (De Juan Ares et al. 2019a). The most striking similarity is the clear prevalence of Levantine glass from the last quarter of the sixth century onwards (Fig. 58b). Sufficient new raw glass from the Levant evidently arrived in Recópolis and literally outlasted the Visigothic period.
Levantine I glass is the largest group until well into the eighth (possibly ninth) century, which may indicate that the imported glass had been securely stored to be processed later. The relative abundance of recycled material increases at Recópolis in the second half of the seventh century, but the degree of recycling is by far not as pronounced as in other places. No heavily recycled Foy 2.2 has been identified, unlike in the rural areas of Gózquez and El Pelícano (De Juan Ares et al. 2019a), in Narbonne (Foy et al. 2003b) and the Crypta Balbi in Rome (Mirti et al. 2000; Mirti et al. 2001). The explanation may be that Recópolis, as an important urban centre with large-scale secondary production facilities, was able to maintain a certain standard in the glass that was used or, more prosaically, that glass-working came to an end as soon as shortages in supplies occurred that would have affected the quality of the glasses, and that are therefore not seen in the archaeological record. Our study of the rural Visigothic sites indicated a notable drop in glass finds from the middle of the seventh century that is not evident from the data at Recópolis, where the absolute numbers remain stable. This, however, may be due to sampling strategies.
The overwhelming presence of Apollonia-type Levantine I glass in Visigothic Spain raises some questions about the supply patterns of glass more generally, because of the virtual absence of Levantine I glass prior to the middle of the sixth century CE. According to Govantes-Edwards and colleagues (2020), the glass assemblage from Recópolis is the exception to the rule in sixth- and seventh-century Spain and inextricably tied to the ideological programme of the Visigothic monarchy, inspired by the Byzantine Empire. Why this should lead to a preference for Levantine glass over Egyptian imports is unclear. Although I agree that more data, especially for the seventh century, are needed for us to be certain about the more fine-grained developments of glass supply in the Iberian Peninsula, the broad trends are beginning to emerge. The last quarter of the sixth century brought a major increase in imports of Apollonia-type Levantine I to the Iberian Peninsula, to inland sites such as Recópolis and the rural settlement of El Pelícano and Gózquez near Madrid (De Juan Ares et al. 2019a) as well as the Visigothic-period settlement of Tolmo de Minateda located about 120 km west of Alicante (Schibille et al. in preparation). Earlier glass assemblages consist almost
exclusively of Egyptian glass groups (HIMT, Foy 2.1), and Egyptian glasses continue to turn up in the archaeological record throughout the latter half of the first millennium (Foy 2.1, Foy 2.2, Magby). The prevalence of Egyptian glass is well illustrated by the finds from the glass factory at the coastal settlement of Benalúa-Alicante. The Benalúa finds form an important glass assemblage that is typologically and compositionally exceedingly homogeneous. Approximately 95% of the glass finds analysed (n=242) are of the Foy 2.1 compositional group with little internal variation, and only a handful of samples have a Levantine I composition (Schibille, unpublished data). This suggests that Levantine glass was not imported on a large scale before the end of the sixth century CE. The spectacular increase in the presence of Levantine I glass at Recópolis thus marks a turning point in the supply patterns of vitreous material in the Iberian Peninsula.
The glass assemblages from Benalúa (sixth century) and Tolmo de Minateda (seventh to ninth centuries) reflect a general surplus of Egyptian glass in the Iberian Peninsula. In Tolmo this includes HIMT, Foy 3.2, Foy 2.1 and Magby glass, that were evidently carefully collected for recycling. With the exception of Magby, these are all older glass production groups (i.e. pre-seventh century), possibly indicating a general decline in imports. At Tolmo de Minateda, we have observed a unique pattern of trace element contamination which must have been introduced into a Foy 2.1 base glass during secondary glass-working or recycling.
About a third of the Foy 2.1 samples exhibit unusually high lithium contents alongside high levels of transition elements. They also have higher rubidium and lower strontium to lime ratios, while the potassium levels do not show any conclusive trend. The glass layers in some crucible fragments that were recovered from Tolmo have exceptionally high lithium levels, too, clearly confirming an increase in contamination as a result of local glass processing. Only some Foy 2.2 samples from the Rabad of Šaqunda (Córdoba) exhibit similar compositional characteristics (Schibille et al. 2020a). Interestingly, elevated lithium levels are typical of Iberian plant ash glass produced during the early Islamic period (e.g. Fig. 64b). It thus seems that the incorporation of some Iberian raw material during the secondary processing of these Foy 2.1 samples foreshadows later developments.
It is unclear how the Arab conquest of Egypt and Syria-Palestine, which was complete by the middle of the seventh century, affected the influx of fresh glass to the Iberian Peninsula. The compositional characteristics of seventh- to eighth-century glass assemblages from most sites that we have studied show an increase in recycling of late antique glass types, while the total number of glass finds is drastically decreasing (De Juan Ares et al. 2019a). The same is true for the period immediately after the Arab conquest of the Iberian Peninsula and the establishment
of al-Andalus in 711 CE. Glass recycling continued well into the eighth century.
A new local production in Córdoba that first appeared in the period between the second half of the eighth and the first quarter of the ninth century changed all that.